Flywheels are employed to store energy as rotational kinetic energy when the production exceeds demand or in machines that require large amounts of energy for short periods of time. In case of the production of electrical energy from wind, for example, it is typical to have excess energy with respect to demand in high wind conditions. For wind farm applications, the excess energy can be stored in a flywheel as rotational kinetic energy and released as electrical energy (power) when the demand becomes larger than the energy (power) produced.
To maximize the amount of stored energy, the moment of inertia and the angular velocity of a flywheel need to be as large as feasible. A flywheel with a fixed moment of inertia has the following characteristics: a) the release of rotational kinetic energy by the flywheel results in a decrease in its angular velocity, and b) the acceleration of the flywheel to the nominal angular velocity takes a long time. These characteristics are undesirable for certain applications partly because of they require output power conditioning. Thus, quickly achieving the nominal angular velocity and maintaining it within a narrow range while storing or releasing energy are desirable. These can be achieved by changing the moment of the inertia of the flywheel during operation. Accordingly, the moment of inertia should be small in the acceleration period, increase to a maximum to store the largest amount of energy possible, and then vary to maintain the nominal angular velocity as close to constant as possible.
Previous solutions of changing the moment of inertia of a flywheel have been based on hydraulic or mechanical systems. Some solutions consist of adding fluid to a hollow flywheel, fluid that is distributed to the outer region by the centrifugal force as described in U.S. Pat. No. 4,335,627 to Maxwell (1982) and U.S. Pat. No. 5,086,664 to Wagner (1992). Other solutions force the fluid between locations along the radius by pistons in cylinders placed radially as shown in U.S. Pat. No. 3,248,967 to Lewis (1966), by pumps between cavities that are placed at two different radii as described in U.S. Pat. No. 6,883,399 to Burstall (2004), or between cells that are arranged to control the mass distribution as presented by U.S. Pat. No. 4,546,264 to Pinson (1985). Mechanical means to vary the moment of inertia include the motion of two articulated counterweights that are moved away from the axis of rotation using a rack and pinion actuator as shown in U.S. Pat. No. 3,863,510 to Benson (1975). U.S. Pat. No. 4,725,766 (1988) and U.S. Pat. No. 4,730,154 (1988) both to Pinson describe the use of masses that can be moved along radial spokes by means of actuators. U.S. Pat. No. 4,926,107 to Pinson (1990) describes several methods to change the moment of inertia by employing either masses that are moved along spokes by actuator or fixed masses that are articulated at the hub in an “umbrella” configuration and swing away radially under the control of their respective drive motors. U.S. Pat. No. 4,995,282 to Schumacher (1999) describes a variable inertia flywheel using two masses that are mechanically pushed along the radial direction. U.S. Pat. No. 7,044,022 to Kim (2006) presents a variable inertia flywheel that is composed of a rotatable member and a body, both containing channels that guide movable masses. The rotatable member can be rotated with respect to the body hydraulically moving the masses along the radial direction. Leung in “IEEE Transactions on Magnetics, Vol. 27, January 1991” describes using a flywheel with variable moment of inertia for pulse conditioning for electromagnetic launch. U.S. Pat. No. 5,531,574 to Hiraishi et al. (1994) describes a variable inertia flywheel that is based on the motion of masses placed in cavities inside the flywheel. The masses are not connected to the cavities with the relative placement between the masses and the flywheel determined by the centrifugal force.
Many of the proposed solutions make the adjustment of the flywheel's moment of inertia by external means. Consequently, they require the input of additional energy into the system as well as separate means of control. This complexity introduces multiple potential points of failure and increases manufacturing and operation costs. Some of the proposed solutions are not designed primarily for efficient energy storage or conversion. Besides, flywheels operate at high velocities, imposing strict safety and durability constrains on the materials, and making implementation of some of the complex designs described in the literature even more challenging. The systems that employ liquids to adjust the moment of inertia do not have a large range of adjustment due to the smaller liquid density which results in a smaller mass Thus, a current need remains for energy storage devices that improve the energy storage and operational characteristics of existing flywheels.